This invention relates in general to obtaining synchronization from a serial bit stream of data pulses, more particularly where the synchronization code is distributed as single bits at fixed bit intervals in the bit stream, such as occurs in telecommunications, especially pulse code modulated telephony systems.
Embedded in the bit stream of some digital transmissions systems, particularly those employed in telephony, is a pre-assigned bit pattern that is distributed at intervals containing a fixed number, N, of bits. It is necessary to search the bit stream to discover the starting point of the embedded pattern so that the meaning of the remaining bits in the stream can be established.
In pulse code modulation telephony, bits are organized in frames, each of which represents the sampled status of all signals associated with that transmission. These samples are normally generated at a rate of 8000 per second. Typically, one bit is added to each frame; and that bit is part of the pre-assigned pattern that signals the start of a frame and, when appropriate, the first frame of a sequence to be established.
Frame synchronization is performed by a number of prior art systems; but these systems have been found to have certain disadvantages. For example, some of the prior art systems are restricted to use with only specific types of framing patterns, such as patterns organized exclusively as alternating ones and zeros, and are not adaptable to the synchronization of alternative patterns of data transmission. Other prior art systems, which may not be pattern dependent, frequently require a substantial amount of hardware to perform the frame sycnchronization function.